It is known that many objects, articles or substances which are relatively pliable, elastic or soft, can be comminuted more effectively in impact-jet or hammer mills as well as in attrition mills such as pin mills, by cooling them to a temperature below their embrittlement point. For example, worn vehicle tires of synthetic and/or natural rubbers cannot be ground in an efficient manner to a granulated or pulverulent mass for subsequent reworking in their normal or soft state.
It has been proposed, therefore, to chill the tires, usually with a liquefied gas (e.g. liquid nitrogen) to embrittle these objects and render them more readily comminutable by attrition or impact mills. Synthetic-resin thermoplastic scraps, e.g. scraps of polyethylene foil, can be recycled in plastic technology in a comminuted state. These scraps are usually too pliable or supple for grinding with conventional apparatus and hence it has been proposed to subject these scraps as well to embrittlement at low and even cryogenic temperatures.
For the most part, such objects are either chilled by permitting them to fall through a cooling tower in counterflow to a rising stream of cooling fluids, by immersing them in a bath of liquefied gas or in some similar manner. In a practical embodiment of these concepts, the chilling apparatus comprises a cooling chamber formed as a tunnel through which the subjects to be chilled are transported on, for example, a conveyor chain, by conveyor baffles, or on conveyor belts. The objects move through the tunnel in counterflow to the cooling fluid which may be in a gaseous state and can leave the tunnel adjacent the object-inlet end. Thus an initial cooling or precooling takes place and the cooling proceeds to lower temperatures as the objects move through the tunnel. A system of this type is described in the German open application (Offenlegungsschrift) DT-OS 22 45 804.
When the objects are large, massive or of complex shape, as is the case with vehicle tires, the cooling tunnel must be very long since the heat transfer coefficient resulting from the gas/solid heat exchange relationship is relatively small, if maximum cold utilization is to be obtained.
Since even with long tunnels it is not always best to completely cool the objects by gas/solid contact, it is a common practice to provide a deep cooling zone in which the transport system carries the objects through a liquefied gas bath. The latter may be provided in a recess or basin at the end of the cooling tunnel and gives a uniform deep cooling of the objects.
The disadvantages of such systems will be immediately apparent. For example, since the transport device must extend through the liquefied gas bath, it must be composed of materials which themselves do not become significantly embrittled at the low temperatures which are employed. Consequently, they are usually constructed from relatively expensive cold-ductile steels. Even when so constructed, they are susceptible to some material fatigue because of the changes in temperature to which they are exposed and breakdown, excessive wear and rupture of the parts of the conveyor are always problems.
Furthermore, the length of the tunnel itself is a significant disadvantage since the transport device must be equally long and hence of complex and expensive construction. Moreover, the ground-space requirement for cooling tunnels renders them uneconomical.